This invention relates to a method for the removal of radioactive iodine from a waste gas stream. More specifically, this invention relates to a method for the removal and long-term storage of radioactive iodine from waste gas streams and particularly from waste gas streams associated with the reprocessing of irradiated nuclear reactor fuel elements.
Of importance to safety in the field of atomic energy power generation is the recovery and storage of radioactive iodines from waste gas streams to prevent dispersion to the environment where they may ultimately be ingested by humans and animal life. These radioactive iodines which include inorganic species such as elemental iodine and organic species such as methyl iodide, may be present in reactor containment vessels from normal reactor operation and in particular in the case of fuel element cladding failure. These iodines are also present in waste gas streams containing off-gases from fuel element dissolver operations in nuclear fuel reprocessing plants. These radioactive iodines are formed in the nuclear reactor fuel by the fission of the fuel material. Of particular interest in addition to .sup.131 I is radioactive iodine .sup.129 I which has a very long half-life and for which long-term storage must therefore be provided.
One method for the recovery of these radioactive iodines is described in U.S. Pat. No. 3,658,467, which issued Apr. 25, 1972 to Maeck, and is assigned to the present assignee. As described in that patent, the waste gas stream containing the radioactive iodine species is passed through a filter bed of synthetic zeolite in a metal ion-exchanged form, the metal being one which is reactive with iodine. The patent suggests the use of a number of metals, such as silver, mercury, thallium, lead and palladium. Of these, the silver metal ion-exchanged zeolite has proven best for adsorbing large amounts of iodine and retaining the iodine under the conditions in which the bed of material must operate.
Silver, however, in addition to being expensive is a valuable resource which is useful for a variety of purposes other than the long-term storage of radioactive iodines. What then is needed is a method for removing the adsorbed iodine from the silver-exchanged zeolite, regenerating the silver-exchanged zeolite and adsorbing the desorbed iodine on a less expensive and more widely available material for long-term storage.